Abstract A critical approach to reducing the national epidemic of opioid-related morbidity and mortality includes maximizing retention rates of opioid agonist therapy (OAT) among individuals with OUD. In particular, OAT using buprenorphine, compared to methadone, may be a suitable starting point for many patients with OUD given its relatively broader safety profile (e.g., lower likelihood of overdose), and increased accessibility and lower associated stigma given that it can be prescribed in office-based medical settings. Nevertheless, among the fraction that do utilize buprenorphine, rates of long-term treatment retention, which are crucial to facilitating best outcomes, are low. Thus, a better understanding of the barriers to sufficient buprenorphine treatment retention are needed in order to inform efforts at optimizing current practice of an effective treatment. One factor common among many patients with OUD that may negatively affect buprenorphine treatment retention is the co-occurring presence of chronic non-cancer pain (CNCP). Current research shows that up to approximately two-thirds of patients with OUD in medical settings, for whom buprenorphine treatment could be applicable, have a co-occurring CNCP condition. However, despite the prevalence of CNCP among patients with OUD, remarkably little is known about its impact on buprenorphine treatment retention. The present R03 proposal will address this gap in the literature through secondary analysis of a large electronic health record database of patients receiving buprenorphine treatment for OUD. The overall goal of this application is to better understand how CNCP may operate as a risk factor for lower buprenorphine retention, and in turn, how buprenorphine treatment can be optimized to improve retention outcomes among patients with co-occurring OUD and CNCP. Specifically, we will first determine the extent to which the impact of CNCP on treatment retention depends on uncontrolled pain during treatment characterized via multi-level growth curve modeling. To supplement these findings, we will conduct retrospective chart reviews to identify the extent to which ongoing pain vs. other reasons were reported for premature treatment discontinuation. Reasons will also be identified by empirically-defined buprenorphine adherence trajectory groups to indicate temporal specificity of discontinuation reasons. Furthermore, we aim to elucidate the association between buprenorphine dose and treatment retention among patients with co-occurring OUD and CNCP, which may represent a modifiable factor that could be targeted to improve retention. Together, the findings from this research may inform targeted efforts to enhance the clinical impact of an already existing treatment for OUD in the face of a rapidly increasing opioid epidemic, which could ultimately lead to greater reductions in opioid-related morbidity and mortality and costly healthcare utilization.